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J. Biol. Chem., Vol. 280, Issue 37, 32141-32147, September 16, 2005

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DNA-induced Secondary Structure of the Carboxyl-terminal Domain of Histone H1^*

Alicia Roque, Ibon Iloro, Imma Ponte, José Luis R. Arrondo, and Pedro Suau1

From the Departamento de Bioquímica y Biología Molecular, Facultad de Ciencias, Universidad Autónoma de Barcelona, 08193 Bellaterra, Barcelona, Spain and the Departamento de Bioquímica, Universidad del País Vasco, Apartado. 644, E-48080, Bilbao, Spain

We have studied the secondary structure of the carboxyl-terminal domains of linker histone H1 subtypes H1⁰ (C-H1⁰) and H1t (C-H1t), free in solution and bound to DNA, by IR spectroscopy. The carboxyl-terminal domain has little structure in aqueous solution but becomes extensively folded upon interaction with DNA. The secondary structure elements present in the bound carboxyl-terminal domain include the -helix, -structure, turns, and open loops. The structure of the bound domain shows a significant dependence on salt concentration. In low salt (10 mM NaCl), there is a residual amount of random coil, 7% in C-H1⁰ and 12% in C-H1t. In physiological salt concentrations (140 mM NaCl), the carboxyl termini become fully structured. Under these conditions, C-H1⁰ contained 24% -helix, 25% -structure, 17% open loops, and 33% turns. The latter component could include a substantial proportion of the 3₁₀ helix. Despite their low sequence identity (30%), the representation of the different structural motifs in C-H1t was similar to that in C-H1⁰. Examination of the changes in the amide I components in the 20–80 °C temperature interval showed that the secondary structure of the DNA-bound C-H1t is for the most part extremely stable. The H1 carboxyl-terminal domain appears to belong to the so-called disordered proteins, undergoing coupled binding and folding.

Received for publication, May 23, 2005 , and in revised form, June 27, 2005.

^* This work was supported by Ministerio de Educación y Ciencia Grants BMC2002-00087 and BMC2002-01438, Generalitat de Catalunya Grant 2001SGR 00199, and University of the Basque Country Grant 00042.310-13552/2001. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed. Tel.: 34-93-5811391; Fax: 34-93-5811264; E-mail: pere.suau{at}uab.es.

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